This application relates to methods and apparatus by which flowable materials, such as particulate materials, are applied to a newly-formed web of paper at the wet end of a paper former. Starch is one such material, and is added to paper fibers to improve strength.
The strength of paper is measured by the well-known parameters of internal bond, tensile, fold, and burst. Such strengths are usually greatest at the machine center and decrease toward the edges of the web. This strength profile is sometimes called a "frown" as it is characterized by an inverted curve as measured in a cross machine profile. It requires that the strength targets be increased in order that all of the paper will meet the minimum strength requirements for the particular paper grade and for its intended use. The decrease or edge fall-off in strength, typically, may be in the order of ten percent, measuring from the center to each of the transverse edges of the web. Therefore, it is frequently necessary to employ an excess strength target in the order of about five to six percent to compensate for the fall-off.
Having to use an excess strength target results in a higher cost to the industry, and practical solutions to this problem are traditionally lacking. Strength may be increased by adding starch to the furnish, but a uniform application of starch does not compensate for the lack of flatness of the strength profile. A need therefore exists for an apparatus and process by means of which the amount of strength adding material, such as starch, may be varied across the width of the web, so that the profile, if desired, may be flattened or so that particular cross machine regions may be enhanced as compared to other regions.